Guide
How to calculate stairs: rise, run and number of steps
By Uri Haramati · Updated November 13, 2026
Measure the total rise, divide it by about 7 1/2 in and round up to get the number of risers. Divide the total rise by that number for the exact riser height. A stair has one fewer tread than risers, so the total run is the number of treads times the tread depth.
What do rise, run, riser and tread mean?
- Total rise: the height from the finished surface at the bottom to the finished surface at the top.
- Total run: the horizontal length of the stair, from the face of the bottom riser to the face of the top riser.
- Riser height: the height of one step.
- Tread depth: the horizontal depth of one step, measured nosing to nosing.
- Stringer: the sloped board, usually a notched 2x12, that carries the treads.
Step 1: Measure the total rise
Measure straight down from the top of the finished deck or floor to the spot where the bottom step will land. If the ground slopes, run a level or a straight board out from the deck and measure down at the landing spot, not at the deck edge. Indoors, include the finished flooring at both ends, since flooring changes the height of the top and bottom steps.
Step 2: Find the number of risers
risers = total rise ÷ 7.5 in, rounded up
7 1/2 in is a comfortable step that sits under the 7 3/4 in maximum in the IRC. Always round up: rounding down makes every step taller than your target.
Step 3: Find the exact riser height
riser height = total rise ÷ risers
Every riser in a flight should be the same. The IRC allows no more than 3/8 in difference between the tallest and the shortest riser in one flight.
Step 4: Choose the tread depth and find the total run
treads = risers − 1
total run = treads × tread depth
The IRC minimum tread depth is 10 in. Many builders use 10 1/2 to 11 in for an easier stair. A quick comfort check is that one riser plus one tread should add up to about 17 to 18 in. If you only have so much room, a shorter run means fewer, taller steps or a landing.
Step 5: Find the stringer length
stringer line = √(total rise² + total run²)
This is the length of the sloped line through the tread corners. Buy a board longer than this to leave room for the cuts at the top and bottom.
Worked example
A deck is 7 ft 11 1/16 in (95 1/16 in) above the pad where the stair will land, and you want 10 1/2 in treads.
| Step | Math | Result |
|---|---|---|
| Number of risers | 95 1/16 ÷ 7.5 = 12.68, round up | 13 risers |
| Riser height | 95 1/16 ÷ 13 | 7 5/16 in |
| Number of treads | 13 − 1 | 12 treads |
| Total run | 12 × 10 1/2 in | 126 in (10 ft 6 in) |
| Comfort check | 7 5/16 + 10 1/2 | 17 13/16 in, within 17 to 18 |
| Stringer line | √(95.06² + 126²) | about 13 ft 1 13/16 in |
The riser is under 7 3/4 in and the tread is over 10 in, so the stair meets the IRC limits for rise and run. See stair code requirements for the rest of the checks.
Common mistakes
- Making as many treads as risers. The top floor is the last step, so there is one fewer tread.
- Measuring the rise at the deck edge on sloped ground. Measure where the bottom step lands.
- Forgetting finished flooring. Flooring at the top or bottom changes the first and last step.
- Forgetting the first-step drop. The stringer is cut short at the bottom by one tread thickness. See how to lay out a stringer.
Skip the math on the job
Stair Once is a stair calculator for iPhone. Type the total rise in feet, inches and sixteenths, or in millimeters, and it gives you the risers, treads, total run and stringer length, checks the stair against the IRC and draws the stringer with its cuts. The calculator is free.
Questions
How many steps do I need for 8 feet of rise?
Divide 96 in by 7 1/2 in to get 12.8, and round up to 13 risers. Each riser is 96 in divided by 13, about 7 3/8 in. A stair with 13 risers has 12 treads.
What is a standard stair rise and run?
Many house stairs use a riser of about 7 to 7 1/2 in and a tread of about 10 1/2 to 11 in. Under the IRC, the riser can be at most 7 3/4 in and the tread at least 10 in.
How do I calculate stairs for a deck?
The method is the same. Measure the total rise from the top of the decking straight down to the pad or ground where the bottom step will land, then divide by about 7 1/2 in and round up to get the number of risers.
Do I count the top floor as a step?
No. The upper floor or deck is the last step you walk onto, so it is not a tread you build. That is why the number of treads is the number of risers minus one.